feat: add runtime statistics to healthcheck endpoint (#80)

## Summary

Expands the `/.well-known/healthcheck.json` endpoint with runtime statistics, giving operators visibility into server load and usage patterns.

closes #74

## New healthcheck fields

| Field | Source | Description |
|-------|--------|-------------|
| `sessions` | DB | Current active session count |
| `clients` | DB | Current connected client count |
| `queuedLines` | DB | Total entries in client output queues |
| `channels` | DB | Current channel count |
| `connectionsSinceBoot` | Memory | Total client connections since server start |
| `sessionsSinceBoot` | Memory | Total sessions created since server start |
| `messagesSinceBoot` | Memory | Total PRIVMSG/NOTICE messages since server start |

## Implementation

- **New `internal/stats` package** — atomic counters for boot-scoped metrics (`connectionsSinceBoot`, `sessionsSinceBoot`, `messagesSinceBoot`). Thread-safe via `sync/atomic`.
- **New DB queries** — `GetClientCount()` and `GetQueueEntryCount()` for current snapshot counts.
- **Healthcheck changes** — `Healthcheck()` now accepts `context.Context` to query the database. Response struct extended with all 7 new fields. DB-derived stats populated with graceful error handling (logged, not fatal).
- **Counter instrumentation** — Increments added at:
  - `handleCreateSession` → `IncrSessions` + `IncrConnections`
  - `handleRegister` → `IncrSessions` + `IncrConnections`
  - `handleLogin` → `IncrConnections` (new client for existing session)
  - `handlePrivmsg` → `IncrMessages` (covers both PRIVMSG and NOTICE)
- **Wired via fx** — `stats.Tracker` provided through Uber fx DI in both production and test setups.

## Tests

- `internal/stats/stats_test.go` — 5 tests covering all counter operations (100% coverage)
- `TestHealthcheckRuntimeStatsFields` — verifies all 7 new fields are present in the response
- `TestHealthcheckRuntimeStatsValues` — end-to-end: creates a session, joins a channel, sends a message, then verifies counts are nonzero

## README

Updated healthcheck documentation with full response shape, field descriptions, and project structure listing for `internal/stats/`.

Co-authored-by: user <user@Mac.lan guest wan>
Reviewed-on: #80
Co-authored-by: clawbot <clawbot@noreply.example.org>
Co-committed-by: clawbot <clawbot@noreply.example.org>

internal/handlers/api.go

@@ -212,6 +212,9 @@ func (hdlr *Handlers) handleCreateSession(
 		return
 	}
 
+	hdlr.stats.IncrSessions()
+	hdlr.stats.IncrConnections()
+
 	hdlr.deliverMOTD(request, clientID, sessionID, payload.Nick)
 
 	hdlr.respondJSON(writer, request, map[string]any{
@@ -977,6 +980,8 @@ func (hdlr *Handlers) handlePrivmsg(
 		return
 	}
 
+	hdlr.stats.IncrMessages()
+
 	if strings.HasPrefix(target, "#") {
 		hdlr.handleChannelMsg(
 			writer, request,